This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to self-crimping fill tube assemblies for use in pressure transmitter systems.
Process instruments are used to monitor process parameters, such as pressure, temperature, flow and level, of fluids used in a variety of industrial processes. For example, process transmitters are used to measure various process fluid parameters situated throughout production facilities such that they can be monitored from a central location. Process transmitters include sensor modules that produce an electrical output in response to physical changes in the process parameter. For example, pressure transmitters may include a capacitive pressure transducer or a piezoresistive pressure transducer that produces an electrical output as a function of the pressure of a process fluid. Output of the sensor module is processed by the transmitter's circuitry such that the sensed parameter can be monitored locally at the transmitter or remotely from a control room. Typically, the pressure of the process fluid is transmitted to the sensor through a closed hydraulic passageway that is in contact with the process fluid through a flexible isolation diaphragm at a first end and a sensor diaphragm at a second end. The accuracy of the output of the pressure transmitter depends on the ability of the closed hydraulic system to convey the magnitude of the process fluid pressure to the sensor module. Typically, the passageway is filled with a precise level of fill fluid that conveys the process fluid pressure from the isolation diaphragm to the sensor diaphragm. In other pressure transmitter systems, remote seal systems serve as extensions of the passageway and comprise a capillary tube having an additional fill fluid. As the pressure of the process fluid fluctuates, the remote seal's fill fluid in series with the transmitter's fill fluid transmits the pressure from the remote seal's isolation diaphragm to the sensor diaphragm, causing the sensor module to alter its electrical output.
The fill fluid passageways in both sensor modules and remote seal systems are filled and sealed during assembly with a precise amount of fill fluid to ensure that the pressure is accurately transmitted from the process fluid to the sensor module. In one method, a fill tube through which the fill fluid is injected into the sensor module extends into the sensor module or remote seal system. After the proper amount of fill fluid is injected, the fill tube can be crimped and welded shut to prevent any fill fluid leakage. The crimp, however, has a tendency to spring-back before the weld takes hold, thus allowing a small amount of fill fluid to escape. The leakage not only reduces the volume of the fill fluid, but contaminates the weld joint, thus making welding difficult and the weld prone to blow-hole formation. Furthermore, in order to perform the crimping and welding procedures, the fill tube must extend from the sensor module or remote seal system an undesirable length that leaves a portion of the fill tube vulnerable to damage. Additionally, crimped and welded fill tubes include no back-up or secondary sealing means, meaning a failure of the weld causes the pressure transmitter to fail to perform. There is, therefore, a need for a crimping assembly that prevents leakage in a more compact and dependable manner.